Project Summary Opioids potently control neuronal circuitry throughout the brain. Endogenous opioid peptides (EOs), produced, packaged, and released by neurons, are synaptically released as the effectors in these pathways. EOs, namely dynorphin and enkephalin, act at the same receptors as exogenous opioids, often by hyperpolarizing target cells and inhibiting neurotransmitter release. The dense expression of dynorphin and enkephalin throughout the hippocampus was first recognized in the 1970s, but functional characterization of endogenous release of these peptides proved difficult due to the limited tools available. As a result, the role of EOs in synaptic and circuit dynamics remains unclear. Furthermore, the opioid peptidergic system undergoes dramatic alterations in epileptogenesis, thus underscoring the need to understand how EOs affect neuronal circuit homeostasis and contribute to disordered network dynamics. My long term goal is to examine the physiology of EO signaling and how it may contribute to epileptic circuits. The proposed experiments will utilize mouse genetics, slice physiology, and novel biosensor technology to interrogate the effects of EO release in the healthy and diseased hippocampus. In Aim 1, I will characterize the pre- and postsynaptic effects of dynorphin and enkephalin signaling at the mossy fiber-CA3 synapse at two target cell types: interneurons and pyramidal cells. Simultaneously, I will visualize opioid release to characterize the spatiotemporal dynamics within the hippocampal circuit. In Aim 2, I will investigate the functional role of dynorphin and enkephalin signaling in hippocampal hyperexcitability. By pharmacologically inducing hyperexcitability in brain slices, I will dissect the roles of enkephalinergic and dynorphinergic pathways in the acutely hyperexcitable circuit. Then, using epileptic mouse models, I will also study the effects of EO signaling in the permanently rearranged circuit. Finally, I will assay the pro- or anti-convulsant effects of evoked EO release in an in vivo seizure susceptibility model. This work will be among the first to explore selective release of EO peptides at hippocampal synapses, and may provide insight regarding the therapeutic potential of EOs in epilepsy. The training to carry out these experiments will develop my scientific competency through close mentorships with highly motivated faculty, as well as coursework to support my development in manuscript writing, grantsmanship, and presentation ability. I will develop proficiency in hypothesis development and experimental design while growing my fund of knowledge in synaptic physiology, circuit research, and translational science. In tandem, I will cultivate leadership and teaching skills within academic medicine, as well as my community, to garner the qualities necessary to be an excellent clinician-scientist.